1. Field of the Invention
The present invention relates to a plasma display panel of an alternating current discharge type (AC-PDP) for use in a flat display capable of easily realizing a larger display area, such as an output display for a personal computer and a work station as well as a wall-mountable TV, and a method for driving the same.
2. Description of the Related Art
PDPs are classified into a DC type and an AC type on the basis of their structures. The DC-PDP includes electrodes that are exposed in a discharge gas. The AC-PDP includes electrodes that are covered with a dielectric material and not exposed directly in the discharge gas. The AC-PDPs are further classified into a memory operation type PDP which employs a memory function by a charge accumulation effect of the dielectric material, and a refresh operation type PDP which does not use that effect.
FIG. 9 is a cross sectional view showing an example of a general AC-PDP structure. The PDP comprises front glass substrate 10 and back glass substrate 11 to form a certain space therebetween for which there is provided the following structure. A plural of scan electrodes 12 and a plural of common electrodes 13, both extending in a direction normal to the drawing and being apart from one another at a certain distance are disposed on front substrate 10. Scan electrodes 12 and common electrodes 13 are covered with insulating layer 15a on which there is formed a protection layer 16 consisting of, for example, MgO for protecting insulating layer 15a from discharge.
A plural of data electrodes 19 extending from left to right on the drawing are disposed on back substrate 11 so as to intercross scan electrodes 12 and common electrodes 13 at right angles. Data electrodes 19 are covered with insulating layer 15b on which there are formed phosphors materials 18 for converting UV rays derived from discharges into visible lights. In order to obtain a color display PDP, each cell may be coated independently with a different phosphors material that has, for example, one of three primary colors of light; red, green and blue (RGB). FIG. 13 shows an example of the coating of phosphors material on each cell, in which R means red, G green and B blue. FIG. 13 depicts arrays in which the phosphors materials of RGBRGB . . . are coated in a row direction and the phosphors materials having the identical light emission colors are coated in a column direction.
Partition 17 for defining discharge space 20 and for separating among cells is located between insulating layer 15a on front substrate 10 and insulating layer 15b on back substrate 11. A discharge gas is enclosed within discharge space 20, which consists of a mixed gas selected from He, Ne, Ar, Kr, Xe N.sub.2, O.sub.2, CO.sub.2 and the like. At least one of substrates 10 and 11 is transparent.
FIG. 10 is a plan view showing an electrode structure in the color PDP shown in FIG. 9. At the electrode structure in the color PDP shown in FIG. 10, m scan electrodes 12 {(S.sub.i (i=1, 2, . . . , m)}) are arranged in a row direction, n data electrodes 19 {D.sub.j (i=1, 2, . . . ,n)} are arranged in a column direction, and thus one cell is provided at a cross point thereof. Common electrodes 13 {(C.sub.i (i=1, 2, . . . , m)} are arranged in the row direction so as to pair with scan electrodes {S.sub.i }, thus both are in parallel to each other.
A conventional method for driving the PDP constructed as above will be explained bellow. FIG. 11 is a timing chart showing drive voltage waveforms applied to each of electrodes in the color PDP shown in FIG. 10.
First, erasing pulses 21 are applied to all the scan electrodes 12 to halt discharge states of cells which have emitted lights till the time shown in FIG. 11 and to bring them into erasing states. The term "erase" herein means an operation of reducing or annihilating wall charges as mentioned later.
Next, priming discharge pulses 22 are applied to common electrodes 13 so that all the cells may emit light by force with discharges, and then priming discharge erasing pulses 23 are applied to scan electrodes 12 in order to erase the priming discharges of all the cells. Priming discharge pulse 22 and priming discharge erasing pulses 23 may ease a write discharge as mentioned later.
After erasing the priming discharge, scan pulses 24 are applied to scan electrodes S.sub.1 -S.sub.m at different timings, and data pulses 27 are applied to data electrodes 19 (D.sub.1 -D.sub.n) in accordance with the timing when the corresponding scan pulse 24 is applied. An oblique line depicted in data pulse 27 shows that presence/absence of data pulse 27 has been determined in accordance with presence/absence of the display data. When applying scan pulses 24, the write discharge may be caused within a discharge space 20 formed between scan electrode 12 and data electrode 19 only in the cells that are provided with data pulses 27, but not in the cells that are not provided with data pulses 27.
Positive charges called wall charges are accumulated on insulating layer 15a on scan electrodes 12 in the cells where there was caused the write discharge. At the same time, negative wall charges are accumulated on insulating layer 15b on data electrodes 19. Superimposing a positive potential due to the positive wall charges, which are generated on insulating layer 15a on scan electrodes 12, onto a first negative sustaining pulse 25, which is applied to common electrodes 13, may cause a first sustaining discharge. When the first sustaining discharge occurs, positive wall charges are accumulated on insulating layer 15a on common electrodes 13, and negative wall charges are accumulated on insulating layer 15a on scan electrodes 12. A second sustaining pulse 26 is superimposed on the potential difference between the wall charges so as to cause a second sustaining discharge. Thus, the potential difference between the wall charges generated by the sustaining discharges of a n-th time may be superimposed on the sustaining pulse of a (n+1)-th time to continue sustaining discharges. The continuation number of the sustaining discharges may control brightness.
If adjusting the voltages of sustaining pulses 25 and 26 previously at such values that can not cause discharges by only these pulse voltages themselves, the potential due to the wall charges is not present in the cells where there were not caused write discharges before applying the first sustaining pulses 25. Therefore, the first sustaining discharges can not occur in such cells even when applying the first sustaining pulses 25, and thus the following sustaining discharges will not occur accordingly. In general, frequencies for applying sustaining pulses 25 and 26 are about 100 kHz, respectively. Waveforms of these pulses are generally rectangular.
In the above explained drive voltage waveforms shown in FIG. 11, the duration for applying erasing pulse 21, priming discharge pulse 22 and priming discharge erasing pulse 23 is called a priming discharge period. The duration for applying scan pulse 24 and data pulse 27 is called a scan period, and the duration for applying sustaining pulses 25 and 26 is called a sustaining period. The priming discharge period, scan period and sustaining period in combination construct a sub-field.
Next, the conventional gradation display method in the PDP will be explained with reference to FIG. 12. A field is duration (for example, 1/60 second) for displaying one scene, and may be divided into a plural of sub-fields (for example, 4 sub-fields). Each sub-field has the configuration shown in FIG. 11 and can be controlled independently of other sub-fields with respect to ON/OFF of display. Each sub-field has a different length of sustaining period or the number of sustaining pulses, and a different brightness accordingly. In the case of 4 divided sub-fields as shown in FIG. 12, by adjusting each sub-field such that a ratio of lengths of sustaining periods, or a ratio of the numbers of sustaining pulses, or a ratio of brightness may come to 1:2:4:8, for example, a display with 16 gradation brightness, which includes brightness ratios of from 0 at the time when all sub-fields are not selected to 15 at the time when all sub-fields are selected, can be achieved in accordance with combinations of display ON/OFF in the sub-field.
Dividing one field into n sub-fields and setting the ratio of lengths of sustaining periods, or the ratio of the numbers of sustaining pulses, or the ratio of brightness per sub-field at 1 (=2.sup.0):2(=2.sup.1): . . . :2.sup.n-2 :2.sup.n-1 may perform 2.sup.n -gradation display.
However, in the case where the conventional method for driving the AC-PDP is employed to display an image, the contrast of the image in the dark place may be greatly affected by the brightness due to the priming discharge operation. This is because, even in the case of the brightness ratio of 0 as is in the darkest light emission state where all sub-fields are not selected, as the light emission due to the priming discharge operation in each sub-filed exists, a complete "black" display can not be obtained. In the conventional driving method, a measured value of brightness for "black" is about 5 cd/m.sup.2, a measured value of brightness for "white" is about 150 cd/m.sup.2, and thus a contrast ratio is about 30:1.
Thus, the conventional AC-PDP includes such a disadvantage that the contrast ratio is low because of high brightness caused by the priming discharge and priming discharge erasing.
JPA-8-221036 discloses a technology for improving the contrast ratio by effecting the priming discharge operation only in a part of sub-field or only in a part of cells. This conventional technology, however, requires an additional signal process for controlling priming discharge and thus complicates the apparatus.
Another method for improving the contrast ratio by introducing priming discharge cells used in DC-PDP into AC-PDP and shading the priming discharge cells is also known. The priming discharge cells are such cells that may only preliminarily discharge independently of the cells for displaying the image.
The conventional priming discharge cells, however, are realized to operate at such locations for causing priming discharge that differ simply from the locations for causing display discharge. Priming discharge operation is one of constituents that consist of sub-field and is not independent of display discharge from a view of driving operation though the locations are independent. Priming discharge is necessary to synchronize with other driving operations such as write discharge and sustaining discharge. This enables to minimize the number of priming discharges. Thus, the conventional technology has a disadvantage that it is necessary to coincide the timings of priming discharge, write discharge and sustaining discharge with one another for adjusting drive waveforms as in the case of a panel structure having no priming discharge cells.